The goal of this study is to understand how hindbrain reticulospinal (RS) neurons control locomotive behaviors in larvae of the teleost fish, Brachydanio rerio (zebrafish). The RS system is known to influence locomotive movements through synaptic connections with spinal neurons, however, the functions of individual RS cell types, and the organization of diverse motor commands within the RS system, are not well understood in any vertebrate species. In fish, the RS system is the primary descending motor tract and therefore must control most locomotor behaviors. This study will investigate the cellular-level control of swimming and turning behaviors in larval zebrafish. In vivo optical methods will be used to measure neural activity in, and to laser ablate, individually identified RS neurons. High-speed digital cinematography will be used to measure kinematic parameters of locomotor movements and to characterize locomotor deficits. Two main goals will be addressed: (l) to identify RS neurons controlling specific motor components of swimming and turning behaviors, and (2) to understand how groups of RS neurons are organized into functional circuits. Because most locomotor behaviors in zebrafish larvae are controlled by only a small number (<200) of RS neurons, this study should yield a more complete understanding of RS locomotor control than currently exists for any other vertebrate animal.